Systems capable of preventing brakes from locking have been in use on vehicles for a number of years. These anti-skid braking systems are commonly referred to as ABS braking system. These systems require a sensor for detecting the speed of rotation of the wheel hub. The sensor element functions to convert mechanical rotation to an electrical signal which is then transmitted to an actuating device. The sensor detects the mechanical signal produced by a wheel known as a "phonic" wheel.
In a typical ABS system, the phonic wheel is usually keyed directly on the outer rotating ring of the wheel hub bearing assembly. This arrangement has certain disadvantages and drawbacks. For example, the phonic wheel is usually an element made of metal which is keyed directly on the outer ring of the wheel hub bearing which is also a metal, such as steel. By reason of the metal to metal contact between the phonic wheel and the outer ring of the bearing, there is a great risk of deformation of the outer ring of the bearing which can damage the entire wheel hub assembly.
Another problem in the prior hub assemblies described above is that in the case of jolts, the phonic wheel may ride on the outer ring and be displaced axially, since there are no interposed elastic elements. This can impair the functioning of the device for detecting the speed of the wheel.
It has also been observed that temperature changes can cause deformations in the system, especially in the phonic wheel which leads to the loss of interference between the sensor and the wheel itself.
It is also the case that the phonic wheel and/or sensor are susceptible to damage in the case of collision between these elements, because their confronting surfaces are metal and are closely adjacent to one another.
Lastly, it has been found that the phonic wheel is subject to corrosion over a period of time by external corroding agents in the environment of the phonic wheel which renders the entire system useless.